internal/graphicscommand: imporve blend log

Updates #2375
Updates #2626
Updates #3083

cmd/ebitenmobile/_files/EbitenSurfaceView.java

@@ -25,10 +25,10 @@ import javax.microedition.khronos.egl.EGLConfig;
 import javax.microedition.khronos.opengles.GL10;
 
 import {{.JavaPkg}}.ebitenmobileview.Ebitenmobileview;
-import {{.JavaPkg}}.ebitenmobileview.RenderRequester;
+import {{.JavaPkg}}.ebitenmobileview.Renderer;
 import {{.JavaPkg}}.{{.PrefixLower}}.EbitenView;
 
-class EbitenSurfaceView extends GLSurfaceView implements RenderRequester {
+class EbitenSurfaceView extends GLSurfaceView implements Renderer {
 
     private class EbitenRenderer implements GLSurfaceView.Renderer {
 
@@ -63,6 +63,10 @@ class EbitenSurfaceView extends GLSurfaceView implements RenderRequester {
                 onceSurfaceCreated_ = true;
                 return;
             }
+            if (hasStrictContextRestoration()) {
+                Ebitenmobileview.onContextLost();
+                return;
+            }
             contextLost_ = true;
             new Handler(Looper.getMainLooper()).post(new Runnable() {
                 @Override
@@ -77,6 +81,8 @@ class EbitenSurfaceView extends GLSurfaceView implements RenderRequester {
         }
     }
 
+    private boolean strictContextRestoration_ = false;
+
     public EbitenSurfaceView(Context context) {
         super(context);
         initialize();
@@ -90,9 +96,11 @@ class EbitenSurfaceView extends GLSurfaceView implements RenderRequester {
     private void initialize() {
         setEGLContextClientVersion(3);
         setEGLConfigChooser(8, 8, 8, 8, 0, 0);
+        // setRenderer must be called before setRenderRequester.
+        // Or, the application crashes.
         setRenderer(new EbitenRenderer());
-        setPreserveEGLContextOnPause(true);
+
-        Ebitenmobileview.setRenderRequester(this);
+        Ebitenmobileview.setRenderer(this);
     }
 
     private void onErrorOnGameUpdate(Exception e) {
@@ -114,6 +122,16 @@ class EbitenSurfaceView extends GLSurfaceView implements RenderRequester {
         }
     }
 
+    @Override
+    public synchronized void setStrictContextRestoration(boolean strictContextRestoration) {
+        strictContextRestoration_ = strictContextRestoration;
+        setPreserveEGLContextOnPause(!strictContextRestoration);
+    }
+
+    private synchronized boolean hasStrictContextRestoration() {
+        return strictContextRestoration_;
+    }
+
     @Override
     public synchronized void requestRenderIfNeeded() {
         if (getRenderMode() == RENDERMODE_WHEN_DIRTY) {

cmd/ebitenmobile/_files/EbitenViewController.m

@@ -20,7 +20,7 @@
 
 #import "Ebitenmobileview.objc.h"
 
-@interface {{.PrefixUpper}}EbitenViewController : UIViewController<EbitenmobileviewRenderRequester, EbitenmobileviewSetGameNotifier>
+@interface {{.PrefixUpper}}EbitenViewController : UIViewController<EbitenmobileviewRenderer, EbitenmobileviewSetGameNotifier>
 @end
 
 @implementation {{.PrefixUpper}}EbitenViewController {
@@ -149,7 +149,7 @@
 
   displayLink_ = [CADisplayLink displayLinkWithTarget:self selector:@selector(drawFrame)];
   [displayLink_ addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
-  EbitenmobileviewSetRenderRequester(self);
+  EbitenmobileviewSetRenderer(self);
 
   // Run the loop. This will never return.
   [[NSRunLoop currentRunLoop] run];
@@ -364,6 +364,10 @@
   }
 }
 
+- (void)setStrictContextRestoration:(BOOL)strictContextRestoration {
+  // Do nothing.
+}
+
 - (void)setExplicitRenderingMode:(BOOL)explicitRendering {
   @synchronized(self) {
     explicitRendering_ = explicitRendering;

gameforui.go

@@ -66,6 +66,10 @@ func (g *gameForUI) NewOffscreenImage(width, height int) *ui.Image {
 	// An image on an atlas is surrounded by a transparent edge,
 	// and the shader program unexpectedly picks the pixel on the edges.
 	imageType := atlas.ImageTypeUnmanaged
+	if ui.Get().IsScreenClearedEveryFrame() {
+		// A volatile image is also always isolated.
+		imageType = atlas.ImageTypeVolatile
+	}
 	g.offscreen = newImage(image.Rect(0, 0, width, height), imageType)
 	return g.offscreen.image
 }

internal/atlas/image.go

@@ -22,11 +22,10 @@ import (
 	"runtime"
 	"sync"
 
-	"github.com/hajimehoshi/ebiten/v2/internal/debug"
 	"github.com/hajimehoshi/ebiten/v2/internal/graphics"
-	"github.com/hajimehoshi/ebiten/v2/internal/graphicscommand"
 	"github.com/hajimehoshi/ebiten/v2/internal/graphicsdriver"
 	"github.com/hajimehoshi/ebiten/v2/internal/packing"
+	"github.com/hajimehoshi/ebiten/v2/internal/restorable"
 	"github.com/hajimehoshi/ebiten/v2/internal/shaderir"
 )
 
@@ -90,11 +89,8 @@ func putImagesOnSourceBackend() {
 // backend is a big texture atlas that can have multiple images.
 // backend is a texture in GPU.
 type backend struct {
-	// image is an atlas on which there might be multiple images.
+	// restorable is an atlas on which there might be multiple images.
-	image *graphicscommand.Image
+	restorable *restorable.Image
-
-	width  int
-	height int
 
 	// page is an atlas map. Each part is called a node.
 	// If page is nil, the backend's image is isolated and not on an atlas.
@@ -121,73 +117,11 @@ func (b *backend) tryAlloc(width, height int) (*packing.Node, bool) {
 		return nil, false
 	}
 
-	b.extendIfNeeded(b.page.Size())
+	b.restorable = b.restorable.Extend(b.page.Size())
 
 	return n, true
 }
 
-// extendIfNeeded extends the image by the given size if necessary.
-// extendIfNeeded creates a new image with the given size and copies the pixels of the given source image.
-// extendIfNeeded disposes an old image after its call when a new image is created.
-func (b *backend) extendIfNeeded(width, height int) {
-	if b.width >= width && b.height >= height {
-		return
-	}
-
-	// Assume that the screen image is never extended.
-	newImg := newClearedImage(width, height, false)
-
-	srcs := [graphics.ShaderSrcImageCount]*graphicscommand.Image{b.image}
-	sw, sh := b.image.InternalSize()
-	vs := make([]float32, 4*graphics.VertexFloatCount)
-	graphics.QuadVerticesFromDstAndSrc(vs, 0, 0, float32(sw), float32(sh), 0, 0, float32(sw), float32(sh), 1, 1, 1, 1)
-	is := graphics.QuadIndices()
-	dr := image.Rect(0, 0, sw, sh)
-	newImg.DrawTriangles(srcs, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, NearestFilterShader.ensureShader(), nil, graphicsdriver.FillRuleFillAll)
-	b.image.Dispose()
-
-	b.image = newImg
-	b.width = width
-	b.height = height
-}
-
-// newClearedImage creates an emtpy image with the given size.
-//
-// Note that Dispose is not called automatically.
-func newClearedImage(width, height int, screen bool) *graphicscommand.Image {
-	i := graphicscommand.NewImage(width, height, screen)
-
-	// This needs to use 'InternalSize' to render the whole region, or edges are unexpectedly cleared on some
-	// devices.
-	iw, ih := i.InternalSize()
-	clearImage(i, image.Rect(0, 0, iw, ih))
-	return i
-}
-
-func clearImage(i *graphicscommand.Image, region image.Rectangle) {
-	vs := make([]float32, 4*graphics.VertexFloatCount)
-	graphics.QuadVerticesFromDstAndSrc(vs, float32(region.Min.X), float32(region.Min.Y), float32(region.Max.X), float32(region.Max.Y), 0, 0, 0, 0, 0, 0, 0, 0)
-	is := graphics.QuadIndices()
-	i.DrawTriangles([graphics.ShaderSrcImageCount]*graphicscommand.Image{}, vs, is, graphicsdriver.BlendClear, region, [graphics.ShaderSrcImageCount]image.Rectangle{}, clearShader.ensureShader(), nil, graphicsdriver.FillRuleFillAll)
-}
-
-func (b *backend) clearPixels(region image.Rectangle) {
-	if region.Dx() <= 0 || region.Dy() <= 0 {
-		panic("atlas: width/height must be positive")
-	}
-	clearImage(b.image, region.Intersect(image.Rect(0, 0, b.width, b.height)))
-}
-
-func (b *backend) writePixels(pixels *graphics.ManagedBytes, region image.Rectangle) {
-	if region.Dx() <= 0 || region.Dy() <= 0 {
-		panic("atlas: width/height must be positive")
-	}
-	if !region.In(image.Rect(0, 0, b.width, b.height)) {
-		panic(fmt.Sprintf("atlas: out of range %v", region))
-	}
-	b.image.WritePixels(pixels, region)
-}
-
 var (
 	// backendsM is a mutex for critical sections of the backend and packing.Node objects.
 	backendsM sync.Mutex
@@ -205,8 +139,6 @@ var (
 
 	imagesUsedAsDestination smallImageSet
 
-	graphicsDriverInitialized bool
-
 	deferred []func()
 
 	// deferredM is a mutex for the slice operations. This must not be used for other usages.
@@ -224,6 +156,8 @@ const (
 	// A screen image is also unmanaged.
 	ImageTypeScreen
 
+	ImageTypeVolatile
+
 	// ImageTypeUnmanaged is an unmanaged image that is not on an atlas.
 	ImageTypeUnmanaged
 )
@@ -242,7 +176,7 @@ type Image struct {
 	// usedAsSourceCount represents how long the image is used as a rendering source and kept not modified with
 	// DrawTriangles.
 	// In the current implementation, if an image is being modified by DrawTriangles, the image is separated from
-	// a graphicscommand.Image on an atlas by ensureIsolatedFromSource.
+	// a restorable image on an atlas by ensureIsolatedFromSource.
 	//
 	// The type is int64 instead of int to avoid overflow when comparing the limitation.
 	//
@@ -433,11 +367,6 @@ func (i *Image) DrawTriangles(srcs [graphics.ShaderSrcImageCount]*Image, vertice
 }
 
 func (i *Image) drawTriangles(srcs [graphics.ShaderSrcImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderSrcImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule) {
-	if len(vertices) == 0 {
-		return
-	}
-
-	// This slice is not escaped to the heap. This can be checked by `go build -gcflags=-m`.
 	backends := make([]*backend, 0, len(srcs))
 	for _, src := range srcs {
 		if src == nil {
@@ -457,7 +386,7 @@ func (i *Image) drawTriangles(srcs [graphics.ShaderSrcImageCount]*Image, vertice
 	for _, src := range srcs {
 		// Compare i and source images after ensuring i is not on an atlas, or
 		// i and a source image might share the same atlas even though i != src.
-		if src != nil && i.backend.image == src.backend.image {
+		if src != nil && i.backend.restorable == src.backend.restorable {
 			panic("atlas: Image.DrawTriangles: source must be different from the receiver")
 		}
 	}
@@ -479,8 +408,8 @@ func (i *Image) drawTriangles(srcs [graphics.ShaderSrcImageCount]*Image, vertice
 			vertices[i+2] += oxf
 			vertices[i+3] += oyf
 		}
-		if shader.ir.Unit == shaderir.Texels {
+		if shader.ensureShader().Unit() == shaderir.Texels {
-			sw, sh := srcs[0].backend.image.InternalSize()
+			sw, sh := srcs[0].backend.restorable.InternalSize()
 			swf, shf := float32(sw), float32(sh)
 			for i := 0; i < n; i += graphics.VertexFloatCount {
 				vertices[i+2] /= swf
@@ -510,15 +439,15 @@ func (i *Image) drawTriangles(srcs [graphics.ShaderSrcImageCount]*Image, vertice
 		srcRegions[i] = srcRegions[i].Add(r.Min)
 	}
 
-	var imgs [graphics.ShaderSrcImageCount]*graphicscommand.Image
+	var imgs [graphics.ShaderSrcImageCount]*restorable.Image
 	for i, src := range srcs {
 		if src == nil {
 			continue
 		}
-		imgs[i] = src.backend.image
+		imgs[i] = src.backend.restorable
 	}
 
-	i.backend.image.DrawTriangles(imgs, vertices, indices, blend, dstRegion, srcRegions, shader.ensureShader(), uniforms, fillRule)
+	i.backend.restorable.DrawTriangles(imgs, vertices, indices, blend, dstRegion, srcRegions, shader.ensureShader(), uniforms, fillRule)
 
 	for _, src := range srcs {
 		if src == nil {
@@ -570,7 +499,7 @@ func (i *Image) writePixels(pix []byte, region image.Rectangle) {
 		region = region.Add(r.Min)
 
 		if pix == nil {
-			i.backend.clearPixels(region)
+			i.backend.restorable.ClearPixels(region)
 			return
 		}
 
@@ -578,7 +507,7 @@ func (i *Image) writePixels(pix []byte, region image.Rectangle) {
 		pix2 := graphics.NewManagedBytes(len(pix), func(bs []byte) {
 			copy(bs, pix)
 		})
-		i.backend.writePixels(pix2, region)
+		i.backend.restorable.WritePixels(pix2, region)
 		return
 	}
 
@@ -607,7 +536,7 @@ func (i *Image) writePixels(pix []byte, region image.Rectangle) {
 			copy(bs[4*j*r.Dx():], pix[4*j*region.Dx():4*(j+1)*region.Dx()])
 		}
 	})
-	i.backend.writePixels(pixb, r)
+	i.backend.restorable.WritePixels(pixb, r)
 }
 
 func (i *Image) ReadPixels(graphicsDriver graphicsdriver.Graphics, pixels []byte, region image.Rectangle) (ok bool, err error) {
@@ -623,29 +552,17 @@ func (i *Image) ReadPixels(graphicsDriver graphicsdriver.Graphics, pixels []byte
 	// To prevent memory leaks, flush the deferred functions here.
 	flushDeferred()
 
-	if err := i.readPixels(graphicsDriver, pixels, region); err != nil {
+	if i.backend == nil || i.backend.restorable == nil {
-		return false, err
+		for i := range pixels {
+			pixels[i] = 0
 		}
 		return true, nil
 	}
 
-func (i *Image) readPixels(graphicsDriver graphicsdriver.Graphics, pixels []byte, region image.Rectangle) error {
+	if err := i.backend.restorable.ReadPixels(graphicsDriver, pixels, region.Add(i.regionWithPadding().Min)); err != nil {
-	if i.backend == nil {
+		return false, err
-		for i := range pixels {
-			pixels[i] = 0
 	}
-		return nil
+	return true, nil
-	}
-
-	if err := i.backend.image.ReadPixels(graphicsDriver, []graphicsdriver.PixelsArgs{
-		{
-			Pixels: pixels,
-			Region: region.Add(i.regionWithPadding().Min),
-		},
-	}); err != nil {
-		return err
-	}
-	return nil
 }
 
 // Deallocate deallocates the internal state.
@@ -686,13 +603,12 @@ func (i *Image) deallocate() {
 		if !i.backend.page.IsEmpty() {
 			// As this part can be reused, this should be cleared explicitly.
 			r := i.regionWithPadding()
-			i.backend.clearPixels(r)
+			i.backend.restorable.ClearPixels(r)
 			return
 		}
 	}
 
-	i.backend.image.Dispose()
+	i.backend.restorable.Dispose()
-	i.backend.image = nil
 
 	for idx, sh := range theBackends {
 		if sh == i.backend {
@@ -735,16 +651,24 @@ func (i *Image) finalize() {
 }
 
 func (i *Image) allocate(forbiddenBackends []*backend, asSource bool) {
-	if !graphicsDriverInitialized {
-		panic("atlas: graphics driver must be ready at allocate but not")
-	}
-
 	if i.backend != nil {
 		panic("atlas: the image is already allocated")
 	}
 
 	runtime.SetFinalizer(i, (*Image).finalize)
 
+	if i.imageType == ImageTypeScreen {
+		if asSource {
+			panic("atlas: a screen image cannot be created as a source")
+		}
+		// A screen image doesn't have a padding.
+		i.backend = &backend{
+			restorable: restorable.NewImage(i.width, i.height, restorable.ImageTypeScreen),
+		}
+		theBackends = append(theBackends, i.backend)
+		return
+	}
+
 	wp := i.width + i.paddingSize()
 	hp := i.height + i.paddingSize()
 
@@ -753,11 +677,13 @@ func (i *Image) allocate(forbiddenBackends []*backend, asSource bool) {
 			panic(fmt.Sprintf("atlas: the image being put on an atlas is too big: width: %d, height: %d", i.width, i.height))
 		}
 
+		typ := restorable.ImageTypeRegular
+		if i.imageType == ImageTypeVolatile {
+			typ = restorable.ImageTypeVolatile
+		}
 		i.backend = &backend{
-			image:  newClearedImage(wp, hp, i.imageType == ImageTypeScreen),
+			restorable: restorable.NewImage(wp, hp, typ),
-			width:  wp,
+			source:     asSource && typ == restorable.ImageTypeRegular,
-			height: hp,
-			source: asSource && i.imageType == ImageTypeRegular,
 		}
 		theBackends = append(theBackends, i.backend)
 		return
@@ -801,10 +727,12 @@ loop:
 		height *= 2
 	}
 
+	typ := restorable.ImageTypeRegular
+	if i.imageType == ImageTypeVolatile {
+		typ = restorable.ImageTypeVolatile
+	}
 	b := &backend{
-		image:  newClearedImage(width, height, false),
+		restorable: restorable.NewImage(width, height, typ),
-		width:  width,
-		height: height,
 		page:       packing.NewPage(width, height, maxSize),
 		source:     asSource,
 	}
@@ -826,7 +754,7 @@ func (i *Image) DumpScreenshot(graphicsDriver graphicsdriver.Graphics, path stri
 		panic("atlas: DumpScreenshots must be called in between BeginFrame and EndFrame")
 	}
 
-	return i.backend.image.Dump(graphicsDriver, path, blackbg, image.Rect(0, 0, i.width, i.height))
+	return i.backend.restorable.Dump(graphicsDriver, path, blackbg, image.Rect(0, 0, i.width, i.height))
 }
 
 func EndFrame() error {
@@ -857,15 +785,7 @@ func SwapBuffers(graphicsDriver graphicsdriver.Graphics) error {
 		}
 	}()
 
-	if debug.IsDebug {
+	if err := restorable.SwapBuffers(graphicsDriver); err != nil {
-		debug.FrameLogf("Internal image sizes:\n")
-		imgs := make([]*graphicscommand.Image, 0, len(theBackends))
-		for _, backend := range theBackends {
-			imgs = append(imgs, backend.image)
-		}
-		graphicscommand.LogImagesInfo(imgs)
-	}
-	if err := graphicscommand.FlushCommands(graphicsDriver, true); err != nil {
 		return err
 	}
 	return nil
@@ -890,7 +810,7 @@ func BeginFrame(graphicsDriver graphicsdriver.Graphics) error {
 
 	var err error
 	initOnce.Do(func() {
-		err = graphicscommand.InitializeGraphicsDriverState(graphicsDriver)
+		err = restorable.InitializeGraphicsDriverState(graphicsDriver)
 		if err != nil {
 			return
 		}
@@ -906,15 +826,18 @@ func BeginFrame(graphicsDriver graphicsdriver.Graphics) error {
 			minDestinationSize = 16
 		}
 		if maxSize == 0 {
-			maxSize = floorPowerOf2(graphicscommand.MaxImageSize(graphicsDriver))
+			maxSize = floorPowerOf2(restorable.MaxImageSize(graphicsDriver))
 		}
-
-		graphicsDriverInitialized = true
 	})
 	if err != nil {
 		return err
 	}
 
+	// Restore images first before other image manipulations (#2075).
+	if err := restorable.RestoreIfNeeded(graphicsDriver); err != nil {
+		return err
+	}
+
 	flushDeferred()
 	putImagesOnSourceBackend()
 
@@ -929,9 +852,5 @@ func DumpImages(graphicsDriver graphicsdriver.Graphics, dir string) (string, err
 		panic("atlas: DumpImages must be called in between BeginFrame and EndFrame")
 	}
 
-	images := make([]*graphicscommand.Image, 0, len(theBackends))
+	return restorable.DumpImages(graphicsDriver, dir)
-	for _, backend := range theBackends {
-		images = append(images, backend.image)
-	}
-	return graphicscommand.DumpImages(images, graphicsDriver, dir)
 }

internal/atlas/image_test.go

@@ -174,8 +174,8 @@ func TestReputOnSourceBackend(t *testing.T) {
 	}
 	img2.WritePixels(pix, image.Rect(0, 0, size, size))
 
-	// Create an unmanaged image. This should always be on a non-source backend.
+	// Create a volatile image. This should always be on a non-source backend.
-	img3 := atlas.NewImage(size, size, atlas.ImageTypeUnmanaged)
+	img3 := atlas.NewImage(size, size, atlas.ImageTypeVolatile)
 	defer img3.Deallocate()
 	img3.WritePixels(make([]byte, 4*size*size), image.Rect(0, 0, size, size))
 	if got, want := img3.IsOnSourceBackendForTesting(), false; got != want {
@@ -685,7 +685,7 @@ func TestImageIsNotReputOnSourceBackendWithoutUsingAsSource(t *testing.T) {
 }
 
 func TestImageWritePixelsModify(t *testing.T) {
-	for _, typ := range []atlas.ImageType{atlas.ImageTypeRegular, atlas.ImageTypeRegular, atlas.ImageTypeUnmanaged} {
+	for _, typ := range []atlas.ImageType{atlas.ImageTypeRegular, atlas.ImageTypeVolatile, atlas.ImageTypeUnmanaged} {
 		const size = 16
 		img := atlas.NewImage(size, size, typ)
 		defer img.Deallocate()

internal/atlas/shader.go

@@ -15,20 +15,15 @@
 package atlas
 
 import (
-	"fmt"
 	"runtime"
 
-	"golang.org/x/sync/errgroup"
+	"github.com/hajimehoshi/ebiten/v2/internal/restorable"
-
-	"github.com/hajimehoshi/ebiten/v2/internal/builtinshader"
-	"github.com/hajimehoshi/ebiten/v2/internal/graphics"
-	"github.com/hajimehoshi/ebiten/v2/internal/graphicscommand"
 	"github.com/hajimehoshi/ebiten/v2/internal/shaderir"
 )
 
 type Shader struct {
 	ir     *shaderir.Program
-	shader *graphicscommand.Shader
+	shader *restorable.Shader
 }
 
 func NewShader(ir *shaderir.Program) *Shader {
@@ -46,11 +41,11 @@ func (s *Shader) finalize() {
 	})
 }
 
-func (s *Shader) ensureShader() *graphicscommand.Shader {
+func (s *Shader) ensureShader() *restorable.Shader {
 	if s.shader != nil {
 		return s.shader
 	}
-	s.shader = graphicscommand.NewShader(s.ir)
+	s.shader = restorable.NewShader(s.ir)
 	runtime.SetFinalizer(s, (*Shader).finalize)
 	return s.shader
 }
@@ -80,38 +75,6 @@ func (s *Shader) deallocate() {
 }
 
 var (
-	NearestFilterShader *Shader
+	NearestFilterShader = &Shader{shader: restorable.NearestFilterShader}
-	LinearFilterShader  *Shader
+	LinearFilterShader  = &Shader{shader: restorable.LinearFilterShader}
-	clearShader         *Shader
 )
-
-func init() {
-	var wg errgroup.Group
-	wg.Go(func() error {
-		ir, err := graphics.CompileShader([]byte(builtinshader.ShaderSource(builtinshader.FilterNearest, builtinshader.AddressUnsafe, false)))
-		if err != nil {
-			return fmt.Errorf("atlas: compiling the nearest shader failed: %w", err)
-		}
-		NearestFilterShader = NewShader(ir)
-		return nil
-	})
-	wg.Go(func() error {
-		ir, err := graphics.CompileShader([]byte(builtinshader.ShaderSource(builtinshader.FilterLinear, builtinshader.AddressUnsafe, false)))
-		if err != nil {
-			return fmt.Errorf("atlas: compiling the linear shader failed: %w", err)
-		}
-		LinearFilterShader = NewShader(ir)
-		return nil
-	})
-	wg.Go(func() error {
-		ir, err := graphics.CompileShader([]byte(builtinshader.ClearShaderSource))
-		if err != nil {
-			return fmt.Errorf("atlas: compiling the clear shader failed: %w", err)
-		}
-		clearShader = NewShader(ir)
-		return nil
-	})
-	if err := wg.Wait(); err != nil {
-		panic(err)
-	}
-}

internal/buffered/image.go

@@ -21,6 +21,7 @@ import (
 	"github.com/hajimehoshi/ebiten/v2/internal/atlas"
 	"github.com/hajimehoshi/ebiten/v2/internal/graphics"
 	"github.com/hajimehoshi/ebiten/v2/internal/graphicsdriver"
+	"github.com/hajimehoshi/ebiten/v2/internal/restorable"
 )
 
 var whiteImage *Image
@@ -46,6 +47,8 @@ type Image struct {
 	// pixels is cached pixels for ReadPixels.
 	// pixels might be out of sync with GPU.
 	// The data of pixels is the secondary data of pixels for ReadPixels.
+	//
+	// pixels is always nil when restorable.AlwaysReadPixelsFromGPU() returns false.
 	pixels []byte
 
 	// pixelsUnsynced represents whether the pixels in CPU and GPU are not synced.
@@ -68,9 +71,6 @@ func (i *Image) Deallocate() {
 }
 
 func (i *Image) ReadPixels(graphicsDriver graphicsdriver.Graphics, pixels []byte, region image.Rectangle) (bool, error) {
-	// Do not call flushDotsBufferIfNeeded here. This would slow (image/draw).Draw.
-	// See ebiten.TestImageDrawOver.
-
 	if region.Dx() == 1 && region.Dy() == 1 {
 		if c, ok := i.dotsBuffer[region.Min]; ok {
 			copy(pixels, c[:])
@@ -78,6 +78,19 @@ func (i *Image) ReadPixels(graphicsDriver graphicsdriver.Graphics, pixels []byte
 		}
 	}
 
+	// If restorable.AlwaysReadPixelsFromGPU() returns false, the pixel data is cached in the restorable package.
+	if !restorable.AlwaysReadPixelsFromGPU() {
+		i.syncPixelsIfNeeded()
+		ok, err := i.img.ReadPixels(graphicsDriver, pixels, region)
+		if err != nil {
+			return false, err
+		}
+		return ok, nil
+	}
+
+	// Do not call syncPixelsIfNeeded here. This would slow (image/draw).Draw.
+	// See ebiten.TestImageDrawOver.
+
 	if i.pixels == nil {
 		pix := make([]byte, 4*i.width*i.height)
 		ok, err := i.img.ReadPixels(graphicsDriver, pix, image.Rect(0, 0, i.width, i.height))

internal/graphicscommand/command.go

@@ -72,14 +72,23 @@ type drawTrianglesCommand struct {
 }
 
 func (c *drawTrianglesCommand) String() string {
-	// TODO: Improve readability
+	var blend string
-	blend := fmt.Sprintf("{src-color: %d, src-alpha: %d, dst-color: %d, dst-alpha: %d, op-color: %d, op-alpha: %d}",
+	switch c.blend {
+	case graphicsdriver.BlendSourceOver:
+		blend = "(source-over)"
+	case graphicsdriver.BlendClear:
+		blend = "(clear)"
+	case graphicsdriver.BlendCopy:
+		blend = "(copy)"
+	default:
+		blend = fmt.Sprintf("{src-rgb: %d, src-alpha: %d, dst-rgb: %d, dst-alpha: %d, op-rgb: %d, op-alpha: %d}",
 			c.blend.BlendFactorSourceRGB,
 			c.blend.BlendFactorSourceAlpha,
 			c.blend.BlendFactorDestinationRGB,
 			c.blend.BlendFactorDestinationAlpha,
 			c.blend.BlendOperationRGB,
 			c.blend.BlendOperationAlpha)
+	}
 
 	dst := fmt.Sprintf("%d", c.dst.id)
 	if c.dst.screen {

internal/restorable/doc.go

@@ -0,0 +1,68 @@
+// Copyright 2017 The Ebiten Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package restorable offers an Image struct that stores image commands
+// and restores its pixel data from the commands when context lost happens.
+//
+// When a function like DrawImage or Fill is called, an Image tries to record
+// the information for restoration.
+//
+// * Context lost
+//
+// Contest lost is a process that information on GPU memory are removed by OS
+// to make more room on GPU memory.
+// This can happen e.g. when GPU memory usage is high, or just switching applications
+// might cause context lost on mobiles.
+// As Ebitengine's image data is on GPU memory, the game can't continue when context lost happens
+// without restoring image information.
+// The package restorable is the package to record information for such restoration.
+//
+// * DrawImage
+//
+// DrawImage function tries to record an item of 'draw image history' in the target image.
+// If a target image is stale or volatile, no item is created.
+// If an item of the history is created,
+// it can be said that the target image depends on the source image.
+// In other words, If A.DrawImage(B, ...) is called,
+// it can be said that the image A depends on the image B.
+//
+// * Fill, WritePixels and Dispose
+//
+// These functions are also drawing functions and the target image stores the pixel data
+// instead of draw image history items. There is no dependency here.
+//
+// * Making images stale
+//
+// After any of the drawing functions is called, the target image can't be depended on by
+// any other images. For example, if an image A depends on an image B, and B is changed
+// by a Fill call after that, the image A can't depend on the image B anymore.
+// In this case, as the image B can no longer be used to restore the image A,
+// the image A becomes 'stale'.
+// As all the stale images are resolved before context lost happens,
+// draw image history items are kept as they are
+// (even if an image C depends on the stale image A, it is still fine).
+//
+// * Stale image
+//
+// A stale image is an image that can't be restored from the recorded information.
+// All stale images must be resolved by reading pixels from GPU before the frame ends.
+// If a source image of DrawImage is a stale image, the target always becomes stale.
+//
+// * Volatile image
+//
+// A volatile image is a special image that is always cleared when a frame starts.
+// For instance, the game screen passed via the update function is a volatile image.
+// A volatile image doesn't have to record the drawing history.
+// If a source image of DrawImage is a volatile image, the target always becomes stale.
+package restorable

internal/restorable/export_test.go

@@ -0,0 +1,28 @@
+// Copyright 2023 The Ebitengine Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package restorable
+
+import (
+	"github.com/hajimehoshi/ebiten/v2/internal/graphicsdriver"
+)
+
+// EnableRestorationForTesting forces to enable restoration for testing.
+func EnableRestorationForTesting() {
+	forceRestoration = true
+}
+
+func ResolveStaleImages(graphicsDriver graphicsdriver.Graphics) error {
+	return resolveStaleImages(graphicsDriver, false)
+}

internal/restorable/image.go

@@ -0,0 +1,736 @@
+// Copyright 2016 The Ebiten Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package restorable
+
+import (
+	"fmt"
+	"image"
+	"sort"
+
+	"github.com/hajimehoshi/ebiten/v2/internal/graphics"
+	"github.com/hajimehoshi/ebiten/v2/internal/graphicscommand"
+	"github.com/hajimehoshi/ebiten/v2/internal/graphicsdriver"
+)
+
+type Pixels struct {
+	pixelsRecords *pixelsRecords
+}
+
+// Apply applies the Pixels state to the given image especially for restoration.
+func (p *Pixels) Apply(img *graphicscommand.Image) {
+	// Pixels doesn't clear the image. This is a caller's responsibility.
+
+	if p.pixelsRecords == nil {
+		return
+	}
+	p.pixelsRecords.apply(img)
+}
+
+func (p *Pixels) AddOrReplace(pix *graphics.ManagedBytes, region image.Rectangle) {
+	if p.pixelsRecords == nil {
+		p.pixelsRecords = &pixelsRecords{}
+	}
+	p.pixelsRecords.addOrReplace(pix, region)
+}
+
+func (p *Pixels) Clear(region image.Rectangle) {
+	// Note that we don't care whether the region is actually removed or not here. There is an actual case that
+	// the region is allocated but nothing is rendered. See TestDisposeImmediately at shareable package.
+	if p.pixelsRecords == nil {
+		return
+	}
+	p.pixelsRecords.clear(region)
+}
+
+func (p *Pixels) ReadPixels(pixels []byte, region image.Rectangle, imageWidth, imageHeight int) {
+	if p.pixelsRecords == nil {
+		for i := range pixels {
+			pixels[i] = 0
+		}
+		return
+	}
+	p.pixelsRecords.readPixels(pixels, region, imageWidth, imageHeight)
+}
+
+func (p *Pixels) AppendRegion(regions []image.Rectangle) []image.Rectangle {
+	if p.pixelsRecords == nil {
+		return regions
+	}
+	return p.pixelsRecords.appendRegions(regions)
+}
+
+func (p *Pixels) Dispose() {
+	if p.pixelsRecords == nil {
+		return
+	}
+	p.pixelsRecords.dispose()
+}
+
+// drawTrianglesHistoryItem is an item for history of draw-image commands.
+type drawTrianglesHistoryItem struct {
+	images     [graphics.ShaderSrcImageCount]*Image
+	vertices   []float32
+	indices    []uint32
+	blend      graphicsdriver.Blend
+	dstRegion  image.Rectangle
+	srcRegions [graphics.ShaderSrcImageCount]image.Rectangle
+	shader     *Shader
+	uniforms   []uint32
+	fillRule   graphicsdriver.FillRule
+}
+
+type ImageType int
+
+const (
+	// ImageTypeRegular indicates the image is a regular image.
+	ImageTypeRegular ImageType = iota
+
+	// ImageTypeScreen indicates the image is used as an actual screen.
+	ImageTypeScreen
+
+	// ImageTypeVolatile indicates the image is cleared whenever a frame starts.
+	//
+	// Regular non-volatile images need to record drawing history or read its pixels from GPU if necessary so that all
+	// the images can be restored automatically from the context lost. However, such recording the drawing history or
+	// reading pixels from GPU are expensive operations. Volatile images can skip such operations, but the image content
+	// is cleared every frame instead.
+	ImageTypeVolatile
+)
+
+// Image represents an image that can be restored when GL context is lost.
+type Image struct {
+	image *graphicscommand.Image
+
+	width  int
+	height int
+
+	basePixels Pixels
+
+	// drawTrianglesHistory is a set of draw-image commands.
+	// TODO: This should be merged with the similar command queue in package graphics (#433).
+	drawTrianglesHistory []*drawTrianglesHistoryItem
+
+	// stale indicates whether the image needs to be synced with GPU as soon as possible.
+	stale bool
+
+	// staleRegions indicates the regions to restore.
+	// staleRegions is valid only when stale is true.
+	// staleRegions is not used when AlwaysReadPixelsFromGPU() returns true.
+	staleRegions []image.Rectangle
+
+	// pixelsCache is cached byte slices for pixels.
+	// pixelsCache is just a cache to avoid allocations (#2375).
+	//
+	// A key is the region and a value is a byte slice for the region.
+	//
+	// It is fine to reuse the same byte slice for the same region for basePixels,
+	// as old pixels for the same region will be invalidated at basePixel.AddOrReplace.
+	pixelsCache map[image.Rectangle][]byte
+
+	// regionsCache is cached regions.
+	// regionsCache is just a cache to avoid allocations (#2375).
+	regionsCache []image.Rectangle
+
+	imageType ImageType
+}
+
+// NewImage creates an emtpy image with the given size.
+//
+// The returned image is cleared.
+//
+// Note that Dispose is not called automatically.
+func NewImage(width, height int, imageType ImageType) *Image {
+	if !graphicsDriverInitialized {
+		panic("restorable: graphics driver must be ready at NewImage but not")
+	}
+
+	i := &Image{
+		image:     graphicscommand.NewImage(width, height, imageType == ImageTypeScreen),
+		width:     width,
+		height:    height,
+		imageType: imageType,
+	}
+
+	// This needs to use 'InternalSize' to render the whole region, or edges are unexpectedly cleared on some
+	// devices.
+	iw, ih := i.image.InternalSize()
+	clearImage(i.image, image.Rect(0, 0, iw, ih))
+	theImages.add(i)
+	return i
+}
+
+// Extend extends the image by the given size.
+// Extend creates a new image with the given size and copies the pixels of the given source image.
+// Extend disposes itself after its call.
+func (i *Image) Extend(width, height int) *Image {
+	if i.width >= width && i.height >= height {
+		return i
+	}
+
+	newImg := NewImage(width, height, i.imageType)
+
+	// Use DrawTriangles instead of WritePixels because the image i might be stale and not have its pixels
+	// information.
+	srcs := [graphics.ShaderSrcImageCount]*Image{i}
+	sw, sh := i.image.InternalSize()
+	vs := make([]float32, 4*graphics.VertexFloatCount)
+	graphics.QuadVerticesFromDstAndSrc(vs, 0, 0, float32(sw), float32(sh), 0, 0, float32(sw), float32(sh), 1, 1, 1, 1)
+	is := graphics.QuadIndices()
+	dr := image.Rect(0, 0, sw, sh)
+	newImg.DrawTriangles(srcs, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, NearestFilterShader, nil, graphicsdriver.FillRuleFillAll)
+	i.Dispose()
+
+	return newImg
+}
+
+func clearImage(i *graphicscommand.Image, region image.Rectangle) {
+	vs := make([]float32, 4*graphics.VertexFloatCount)
+	graphics.QuadVerticesFromDstAndSrc(vs, float32(region.Min.X), float32(region.Min.Y), float32(region.Max.X), float32(region.Max.Y), 0, 0, 0, 0, 0, 0, 0, 0)
+	is := graphics.QuadIndices()
+	i.DrawTriangles([graphics.ShaderSrcImageCount]*graphicscommand.Image{}, vs, is, graphicsdriver.BlendClear, region, [graphics.ShaderSrcImageCount]image.Rectangle{}, clearShader.shader, nil, graphicsdriver.FillRuleFillAll)
+}
+
+// BasePixelsForTesting returns the image's basePixels for testing.
+func (i *Image) BasePixelsForTesting() *Pixels {
+	return &i.basePixels
+}
+
+// makeStale makes the image stale.
+func (i *Image) makeStale(rect image.Rectangle) {
+	i.stale = true
+
+	// If ReadPixels always reads pixels from GPU, staleRegions are never used.
+	if AlwaysReadPixelsFromGPU() {
+		return
+	}
+
+	origSize := len(i.staleRegions)
+	i.staleRegions = i.appendRegionsForDrawTriangles(i.staleRegions)
+	if !rect.Empty() {
+		i.staleRegions = append(i.staleRegions, rect)
+	}
+
+	i.clearDrawTrianglesHistory()
+
+	// Clear pixels to save memory.
+	for _, r := range i.staleRegions[origSize:] {
+		i.basePixels.Clear(r)
+	}
+
+	// Don't have to call makeStale recursively here.
+	// Restoration is done after topological sorting is done.
+	// If an image depends on another stale image, this means that
+	// the former image can be restored from the latest state of the latter image.
+}
+
+// ClearPixels clears the specified region by WritePixels.
+func (i *Image) ClearPixels(region image.Rectangle) {
+	i.WritePixels(nil, region)
+}
+
+func (i *Image) needsRestoration() bool {
+	return i.imageType == ImageTypeRegular
+}
+
+// WritePixels replaces the image pixels with the given pixels slice.
+//
+// The specified region must not be overlapped with other regions by WritePixels.
+func (i *Image) WritePixels(pixels *graphics.ManagedBytes, region image.Rectangle) {
+	if region.Dx() <= 0 || region.Dy() <= 0 {
+		panic("restorable: width/height must be positive")
+	}
+	w, h := i.width, i.height
+	if !region.In(image.Rect(0, 0, w, h)) {
+		panic(fmt.Sprintf("restorable: out of range %v", region))
+	}
+
+	// TODO: Avoid making other images stale if possible. (#514)
+	// For this purpose, images should remember which part of that is used for DrawTriangles.
+	theImages.makeStaleIfDependingOn(i)
+
+	if pixels != nil {
+		i.image.WritePixels(pixels, region)
+	} else {
+		clearImage(i.image, region)
+	}
+
+	// Even if the image is already stale, call makeStale to extend the stale region.
+	if !needsRestoration() || !i.needsRestoration() || i.stale {
+		i.makeStale(region)
+		return
+	}
+
+	if region.Eq(image.Rect(0, 0, w, h)) {
+		if pixels != nil {
+			// Clone a ManagedBytes as the package graphicscommand has a different lifetime management.
+			i.basePixels.AddOrReplace(pixels.Clone(), image.Rect(0, 0, w, h))
+		} else {
+			i.basePixels.Clear(image.Rect(0, 0, w, h))
+		}
+		i.clearDrawTrianglesHistory()
+		i.stale = false
+		i.staleRegions = i.staleRegions[:0]
+		return
+	}
+
+	// Records for DrawTriangles cannot come before records for WritePixels.
+	if len(i.drawTrianglesHistory) > 0 {
+		i.makeStale(region)
+		return
+	}
+
+	if pixels != nil {
+		// Clone a ManagedBytes as the package graphicscommand has a different lifetime management.
+		i.basePixels.AddOrReplace(pixels.Clone(), region)
+	} else {
+		i.basePixels.Clear(region)
+	}
+}
+
+// DrawTriangles draws triangles with the given image.
+//
+// The vertex floats are:
+//
+//	0: Destination X in pixels
+//	1: Destination Y in pixels
+//	2: Source X in texels
+//	3: Source Y in texels
+//	4: Color R [0.0-1.0]
+//	5: Color G
+//	6: Color B
+//	7: Color Y
+func (i *Image) DrawTriangles(srcs [graphics.ShaderSrcImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderSrcImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule) {
+	if len(vertices) == 0 {
+		return
+	}
+	theImages.makeStaleIfDependingOn(i)
+
+	// TODO: Add tests to confirm this logic.
+	var srcstale bool
+	for _, src := range srcs {
+		if src == nil {
+			continue
+		}
+		if src.stale || src.imageType == ImageTypeVolatile {
+			srcstale = true
+			break
+		}
+	}
+
+	// Even if the image is already stale, call makeStale to extend the stale region.
+	if srcstale || !needsRestoration() || !i.needsRestoration() || i.stale {
+		i.makeStale(dstRegion)
+	} else {
+		i.appendDrawTrianglesHistory(srcs, vertices, indices, blend, dstRegion, srcRegions, shader, uniforms, fillRule)
+	}
+
+	var imgs [graphics.ShaderSrcImageCount]*graphicscommand.Image
+	for i, src := range srcs {
+		if src == nil {
+			continue
+		}
+		imgs[i] = src.image
+	}
+	i.image.DrawTriangles(imgs, vertices, indices, blend, dstRegion, srcRegions, shader.shader, uniforms, fillRule)
+}
+
+// appendDrawTrianglesHistory appends a draw-image history item to the image.
+func (i *Image) appendDrawTrianglesHistory(srcs [graphics.ShaderSrcImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderSrcImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule) {
+	if i.stale || !i.needsRestoration() {
+		panic("restorable: an image must not be stale or need restoration at appendDrawTrianglesHistory")
+	}
+	if AlwaysReadPixelsFromGPU() {
+		panic("restorable: appendDrawTrianglesHistory must not be called when AlwaysReadPixelsFromGPU() returns true")
+	}
+
+	// TODO: Would it be possible to merge draw image history items?
+	const maxDrawTrianglesHistoryCount = 1024
+	if len(i.drawTrianglesHistory)+1 > maxDrawTrianglesHistoryCount {
+		i.makeStale(dstRegion)
+		return
+	}
+	// All images must be resolved and not stale each after frame.
+	// So we don't have to care if image is stale or not here.
+
+	vs := make([]float32, len(vertices))
+	copy(vs, vertices)
+
+	is := make([]uint32, len(indices))
+	copy(is, indices)
+
+	us := make([]uint32, len(uniforms))
+	copy(us, uniforms)
+
+	item := &drawTrianglesHistoryItem{
+		images:     srcs,
+		vertices:   vs,
+		indices:    is,
+		blend:      blend,
+		dstRegion:  dstRegion,
+		srcRegions: srcRegions,
+		shader:     shader,
+		uniforms:   us,
+		fillRule:   fillRule,
+	}
+	i.drawTrianglesHistory = append(i.drawTrianglesHistory, item)
+}
+
+func (i *Image) readPixelsFromGPUIfNeeded(graphicsDriver graphicsdriver.Graphics) error {
+	if len(i.drawTrianglesHistory) > 0 || i.stale {
+		if err := i.readPixelsFromGPU(graphicsDriver); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (i *Image) ReadPixels(graphicsDriver graphicsdriver.Graphics, pixels []byte, region image.Rectangle) error {
+	if AlwaysReadPixelsFromGPU() {
+		if err := i.image.ReadPixels(graphicsDriver, []graphicsdriver.PixelsArgs{
+			{
+				Pixels: pixels,
+				Region: region,
+			},
+		}); err != nil {
+			return err
+		}
+		return nil
+	}
+
+	if err := i.readPixelsFromGPUIfNeeded(graphicsDriver); err != nil {
+		return err
+	}
+	if got, want := len(pixels), 4*region.Dx()*region.Dy(); got != want {
+		return fmt.Errorf("restorable: len(pixels) must be %d but %d at ReadPixels", want, got)
+	}
+	i.basePixels.ReadPixels(pixels, region, i.width, i.height)
+	return nil
+}
+
+// makeStaleIfDependingOn makes the image stale if the image depends on target.
+func (i *Image) makeStaleIfDependingOn(target *Image) {
+	if i.stale {
+		return
+	}
+	if i.dependsOn(target) {
+		// There is no new region to make stale.
+		i.makeStale(image.Rectangle{})
+	}
+}
+
+// makeStaleIfDependingOnShader makes the image stale if the image depends on shader.
+func (i *Image) makeStaleIfDependingOnShader(shader *Shader) {
+	if i.stale {
+		return
+	}
+	if i.dependsOnShader(shader) {
+		// There is no new region to make stale.
+		i.makeStale(image.Rectangle{})
+	}
+}
+
+// readPixelsFromGPU reads the pixels from GPU and resolves the image's 'stale' state.
+func (i *Image) readPixelsFromGPU(graphicsDriver graphicsdriver.Graphics) error {
+	var rs []image.Rectangle
+	if i.stale {
+		rs = i.staleRegions
+	} else {
+		i.regionsCache = i.appendRegionsForDrawTriangles(i.regionsCache)
+		defer func() {
+			i.regionsCache = i.regionsCache[:0]
+		}()
+		rs = i.regionsCache
+	}
+
+	// Remove duplications. Is this heavy?
+	rs = rs[:removeDuplicatedRegions(rs)]
+
+	args := make([]graphicsdriver.PixelsArgs, 0, len(rs))
+	for _, r := range rs {
+		if r.Empty() {
+			continue
+		}
+
+		if i.pixelsCache == nil {
+			i.pixelsCache = map[image.Rectangle][]byte{}
+		}
+
+		pix, ok := i.pixelsCache[r]
+		if !ok {
+			pix = make([]byte, 4*r.Dx()*r.Dy())
+			i.pixelsCache[r] = pix
+		}
+
+		args = append(args, graphicsdriver.PixelsArgs{
+			Pixels: pix,
+			Region: r,
+		})
+	}
+
+	if err := i.image.ReadPixels(graphicsDriver, args); err != nil {
+		return err
+	}
+
+	for _, a := range args {
+		bs := graphics.NewManagedBytes(len(a.Pixels), func(bs []byte) {
+			copy(bs, a.Pixels)
+		})
+		i.basePixels.AddOrReplace(bs, a.Region)
+	}
+
+	i.clearDrawTrianglesHistory()
+	i.stale = false
+	i.staleRegions = i.staleRegions[:0]
+	return nil
+}
+
+// resolveStale resolves the image's 'stale' state.
+func (i *Image) resolveStale(graphicsDriver graphicsdriver.Graphics) error {
+	if !needsRestoration() {
+		return nil
+	}
+	if !i.needsRestoration() {
+		return nil
+	}
+	if !i.stale {
+		return nil
+	}
+	return i.readPixelsFromGPU(graphicsDriver)
+}
+
+// dependsOn reports whether the image depends on target.
+func (i *Image) dependsOn(target *Image) bool {
+	for _, c := range i.drawTrianglesHistory {
+		for _, img := range c.images {
+			if img == nil {
+				continue
+			}
+			if img == target {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+// dependsOnShader reports whether the image depends on shader.
+func (i *Image) dependsOnShader(shader *Shader) bool {
+	for _, c := range i.drawTrianglesHistory {
+		if c.shader == shader {
+			return true
+		}
+	}
+	return false
+}
+
+// dependingImages returns all images that is depended on the image.
+func (i *Image) dependingImages() map[*Image]struct{} {
+	r := map[*Image]struct{}{}
+	for _, c := range i.drawTrianglesHistory {
+		for _, img := range c.images {
+			if img == nil {
+				continue
+			}
+			r[img] = struct{}{}
+		}
+	}
+	return r
+}
+
+// hasDependency returns a boolean value indicating whether the image depends on another image.
+func (i *Image) hasDependency() bool {
+	if i.stale {
+		return false
+	}
+	return len(i.drawTrianglesHistory) > 0
+}
+
+// Restore restores *graphicscommand.Image from the pixels using its state.
+func (i *Image) restore(graphicsDriver graphicsdriver.Graphics) error {
+	w, h := i.width, i.height
+	// Do not dispose the image here. The image should be already disposed.
+
+	switch i.imageType {
+	case ImageTypeScreen:
+		// The screen image should also be recreated because framebuffer might
+		// be changed.
+		i.image = graphicscommand.NewImage(w, h, true)
+		i.basePixels.Dispose()
+		i.basePixels = Pixels{}
+		i.clearDrawTrianglesHistory()
+		i.stale = false
+		i.staleRegions = i.staleRegions[:0]
+		return nil
+	case ImageTypeVolatile:
+		i.image = graphicscommand.NewImage(w, h, false)
+		iw, ih := i.image.InternalSize()
+		clearImage(i.image, image.Rect(0, 0, iw, ih))
+		return nil
+	}
+
+	if i.stale {
+		panic("restorable: pixels must not be stale when restoring")
+	}
+
+	gimg := graphicscommand.NewImage(w, h, false)
+	// Clear the image explicitly.
+	iw, ih := gimg.InternalSize()
+	clearImage(gimg, image.Rect(0, 0, iw, ih))
+
+	i.basePixels.Apply(gimg)
+
+	for _, c := range i.drawTrianglesHistory {
+		var imgs [graphics.ShaderSrcImageCount]*graphicscommand.Image
+		for i, img := range c.images {
+			if img == nil {
+				continue
+			}
+			if img.hasDependency() {
+				panic("restorable: all dependencies must be already resolved but not")
+			}
+			imgs[i] = img.image
+		}
+		gimg.DrawTriangles(imgs, c.vertices, c.indices, c.blend, c.dstRegion, c.srcRegions, c.shader.shader, c.uniforms, c.fillRule)
+	}
+
+	// In order to clear the draw-triangles history, read pixels from GPU.
+	if len(i.drawTrianglesHistory) > 0 {
+		i.regionsCache = i.appendRegionsForDrawTriangles(i.regionsCache)
+		defer func() {
+			i.regionsCache = i.regionsCache[:0]
+		}()
+
+		args := make([]graphicsdriver.PixelsArgs, 0, len(i.regionsCache))
+		for _, r := range i.regionsCache {
+			if r.Empty() {
+				continue
+			}
+
+			if i.pixelsCache == nil {
+				i.pixelsCache = map[image.Rectangle][]byte{}
+			}
+
+			pix, ok := i.pixelsCache[r]
+			if !ok {
+				pix = make([]byte, 4*r.Dx()*r.Dy())
+				i.pixelsCache[r] = pix
+			}
+			args = append(args, graphicsdriver.PixelsArgs{
+				Pixels: pix,
+				Region: r,
+			})
+		}
+
+		if err := gimg.ReadPixels(graphicsDriver, args); err != nil {
+			return err
+		}
+
+		for _, a := range args {
+			bs := graphics.NewManagedBytes(len(a.Pixels), func(bs []byte) {
+				copy(bs, a.Pixels)
+			})
+			i.basePixels.AddOrReplace(bs, a.Region)
+		}
+	}
+
+	i.image = gimg
+	i.clearDrawTrianglesHistory()
+	i.stale = false
+	i.staleRegions = i.staleRegions[:0]
+	return nil
+}
+
+// Dispose disposes the image.
+//
+// After disposing, calling the function of the image causes unexpected results.
+func (i *Image) Dispose() {
+	theImages.remove(i)
+	i.image.Dispose()
+	i.image = nil
+	i.basePixels.Dispose()
+	i.basePixels = Pixels{}
+	i.pixelsCache = nil
+	i.clearDrawTrianglesHistory()
+	i.stale = false
+	i.staleRegions = i.staleRegions[:0]
+}
+
+func (i *Image) Dump(graphicsDriver graphicsdriver.Graphics, path string, blackbg bool, rect image.Rectangle) (string, error) {
+	return i.image.Dump(graphicsDriver, path, blackbg, rect)
+}
+
+func (i *Image) clearDrawTrianglesHistory() {
+	// Clear the items explicitly, or the references might remain (#1803).
+	for idx := range i.drawTrianglesHistory {
+		i.drawTrianglesHistory[idx] = nil
+	}
+	i.drawTrianglesHistory = i.drawTrianglesHistory[:0]
+}
+
+func (i *Image) InternalSize() (int, int) {
+	return i.image.InternalSize()
+}
+
+func (i *Image) appendRegionsForDrawTriangles(regions []image.Rectangle) []image.Rectangle {
+	for _, d := range i.drawTrianglesHistory {
+		if d.dstRegion.Empty() {
+			continue
+		}
+		regions = append(regions, d.dstRegion)
+	}
+	return regions
+}
+
+// removeDuplicatedRegions removes duplicated regions and returns a shrunk slice.
+// If a region covers preceding regions, the covered regions are removed.
+func removeDuplicatedRegions(regions []image.Rectangle) int {
+	// Sweep and prune algorithm
+
+	sort.Slice(regions, func(i, j int) bool {
+		return regions[i].Min.X < regions[j].Min.X
+	})
+
+	for i, r := range regions {
+		if r.Empty() {
+			continue
+		}
+		for j := i + 1; j < len(regions); j++ {
+			rr := regions[j]
+			if rr.Empty() {
+				continue
+			}
+			if r.Max.X <= rr.Min.X {
+				break
+			}
+			if rr.In(r) {
+				regions[j] = image.Rectangle{}
+			} else if r.In(rr) {
+				regions[i] = image.Rectangle{}
+				break
+			}
+		}
+	}
+
+	var n int
+	for _, r := range regions {
+		if r.Empty() {
+			continue
+		}
+		regions[n] = r
+		n++
+	}
+
+	return n
+}

internal/restorable/images.go

@@ -0,0 +1,291 @@
+// Copyright 2017 The Ebiten Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package restorable
+
+import (
+	"image"
+	"runtime"
+	"sync"
+	"sync/atomic"
+
+	"github.com/hajimehoshi/ebiten/v2/internal/debug"
+	"github.com/hajimehoshi/ebiten/v2/internal/graphicscommand"
+	"github.com/hajimehoshi/ebiten/v2/internal/graphicsdriver"
+)
+
+// forceRestoration reports whether restoration forcibly happens or not.
+// This is used only for testing.
+var forceRestoration = false
+
+// disabled indicates that restoration is disabled or not.
+// Restoration is enabled by default for some platforms like Android for safety.
+// Before SetGame, it is not possible to determine whether restoration is needed or not.
+var disabled atomic.Bool
+
+var disabledOnce sync.Once
+
+// Disable disables restoration.
+func Disable() {
+	disabled.Store(true)
+}
+
+// needsRestoration reports whether restoration process works or not.
+func needsRestoration() bool {
+	if forceRestoration {
+		return true
+	}
+	// TODO: If Vulkan is introduced, restoration might not be needed.
+	if runtime.GOOS == "android" {
+		return !disabled.Load()
+	}
+	return false
+}
+
+// AlwaysReadPixelsFromGPU reports whether ReadPixels always reads pixels from GPU or not.
+func AlwaysReadPixelsFromGPU() bool {
+	return !needsRestoration()
+}
+
+// images is a set of Image objects.
+type images struct {
+	images      map[*Image]struct{}
+	shaders     map[*Shader]struct{}
+	lastTarget  *Image
+	contextLost atomic.Bool
+}
+
+// theImages represents the images for the current process.
+var theImages = &images{
+	images:  map[*Image]struct{}{},
+	shaders: map[*Shader]struct{}{},
+}
+
+func SwapBuffers(graphicsDriver graphicsdriver.Graphics) error {
+	if debug.IsDebug {
+		debug.FrameLogf("Internal image sizes:\n")
+		imgs := make([]*graphicscommand.Image, 0, len(theImages.images))
+		for i := range theImages.images {
+			imgs = append(imgs, i.image)
+		}
+		graphicscommand.LogImagesInfo(imgs)
+	}
+	return resolveStaleImages(graphicsDriver, true)
+}
+
+// resolveStaleImages flushes the queued draw commands and resolves all stale images.
+// If endFrame is true, the current screen might be used to present when flushing the commands.
+func resolveStaleImages(graphicsDriver graphicsdriver.Graphics, endFrame bool) error {
+	// When Disable is called, all the images data should be evicted once.
+	if disabled.Load() {
+		disabledOnce.Do(func() {
+			for img := range theImages.images {
+				img.makeStale(image.Rectangle{})
+			}
+		})
+	}
+
+	if err := graphicscommand.FlushCommands(graphicsDriver, endFrame); err != nil {
+		return err
+	}
+	if !needsRestoration() {
+		return nil
+	}
+	return theImages.resolveStaleImages(graphicsDriver)
+}
+
+// RestoreIfNeeded restores the images.
+//
+// Restoration means to make all *graphicscommand.Image objects have their textures and framebuffers.
+func RestoreIfNeeded(graphicsDriver graphicsdriver.Graphics) error {
+	if !needsRestoration() {
+		return nil
+	}
+
+	if !forceRestoration && !theImages.contextLost.Load() {
+		return nil
+	}
+
+	if err := graphicscommand.ResetGraphicsDriverState(graphicsDriver); err != nil {
+		return err
+	}
+	return theImages.restore(graphicsDriver)
+}
+
+// DumpImages dumps all the current images to the specified directory.
+//
+// This is for testing usage.
+func DumpImages(graphicsDriver graphicsdriver.Graphics, dir string) (string, error) {
+	images := make([]*graphicscommand.Image, 0, len(theImages.images))
+	for img := range theImages.images {
+		images = append(images, img.image)
+	}
+
+	return graphicscommand.DumpImages(images, graphicsDriver, dir)
+}
+
+// add adds img to the images.
+func (i *images) add(img *Image) {
+	i.images[img] = struct{}{}
+}
+
+func (i *images) addShader(shader *Shader) {
+	i.shaders[shader] = struct{}{}
+}
+
+// remove removes img from the images.
+func (i *images) remove(img *Image) {
+	i.makeStaleIfDependingOn(img)
+	delete(i.images, img)
+}
+
+func (i *images) removeShader(shader *Shader) {
+	i.makeStaleIfDependingOnShader(shader)
+	delete(i.shaders, shader)
+}
+
+// resolveStaleImages resolves stale images.
+func (i *images) resolveStaleImages(graphicsDriver graphicsdriver.Graphics) error {
+	i.lastTarget = nil
+	for img := range i.images {
+		if err := img.resolveStale(graphicsDriver); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// makeStaleIfDependingOn makes all the images stale that depend on target.
+//
+// When target is modified, all images depending on target can't be restored with target.
+// makeStaleIfDependingOn is called in such situation.
+func (i *images) makeStaleIfDependingOn(target *Image) {
+	if target == nil {
+		panic("restorable: target must not be nil at makeStaleIfDependingOn")
+	}
+	if i.lastTarget == target {
+		return
+	}
+	i.lastTarget = target
+	for img := range i.images {
+		img.makeStaleIfDependingOn(target)
+	}
+}
+
+// makeStaleIfDependingOn makes all the images stale that depend on shader.
+func (i *images) makeStaleIfDependingOnShader(shader *Shader) {
+	if shader == nil {
+		panic("restorable: shader must not be nil at makeStaleIfDependingOnShader")
+	}
+	for img := range i.images {
+		img.makeStaleIfDependingOnShader(shader)
+	}
+}
+
+// restore restores the images.
+//
+// Restoration means to make all *graphicscommand.Image objects have their textures and framebuffers.
+func (i *images) restore(graphicsDriver graphicsdriver.Graphics) error {
+	if !needsRestoration() {
+		panic("restorable: restore cannot be called when restoration is disabled")
+	}
+
+	// Dispose all the shaders ahead of restoration. A current shader ID and a new shader ID can be duplicated.
+	for s := range i.shaders {
+		s.shader.Dispose()
+		s.shader = nil
+	}
+	for s := range i.shaders {
+		s.restore()
+	}
+
+	// Dispose all the images ahead of restoration. A current texture ID and a new texture ID can be duplicated.
+	// TODO: Write a test to confirm that ID duplication never happens.
+	for i := range i.images {
+		i.image.Dispose()
+		i.image = nil
+	}
+
+	// Let's do topological sort based on dependencies of drawing history.
+	// It is assured that there are not loops since cyclic drawing makes images stale.
+	type edge struct {
+		source *Image
+		target *Image
+	}
+	images := map[*Image]struct{}{}
+	for i := range i.images {
+		images[i] = struct{}{}
+	}
+	edges := map[edge]struct{}{}
+	for t := range images {
+		for s := range t.dependingImages() {
+			edges[edge{source: s, target: t}] = struct{}{}
+		}
+	}
+
+	var sorted []*Image
+	for len(images) > 0 {
+		// current represents images that have no incoming edges.
+		current := map[*Image]struct{}{}
+		for i := range images {
+			current[i] = struct{}{}
+		}
+		for e := range edges {
+			if _, ok := current[e.target]; ok {
+				delete(current, e.target)
+			}
+		}
+		for i := range current {
+			delete(images, i)
+			sorted = append(sorted, i)
+		}
+		removed := []edge{}
+		for e := range edges {
+			if _, ok := current[e.source]; ok {
+				removed = append(removed, e)
+			}
+		}
+		for _, e := range removed {
+			delete(edges, e)
+		}
+	}
+
+	for _, img := range sorted {
+		if err := img.restore(graphicsDriver); err != nil {
+			return err
+		}
+	}
+
+	i.contextLost.Store(false)
+
+	return nil
+}
+
+var graphicsDriverInitialized bool
+
+// InitializeGraphicsDriverState initializes the graphics driver state.
+func InitializeGraphicsDriverState(graphicsDriver graphicsdriver.Graphics) error {
+	graphicsDriverInitialized = true
+	return graphicscommand.InitializeGraphicsDriverState(graphicsDriver)
+}
+
+// MaxImageSize returns the maximum size of an image.
+func MaxImageSize(graphicsDriver graphicsdriver.Graphics) int {
+	return graphicscommand.MaxImageSize(graphicsDriver)
+}
+
+// OnContextLost is called when the context lost is detected in an explicit way.
+func OnContextLost() {
+	theImages.contextLost.Store(true)
+}

internal/restorable/pixelrecords.go

@@ -0,0 +1,159 @@
+// Copyright 2019 The Ebiten Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package restorable
+
+import (
+	"fmt"
+	"image"
+
+	"github.com/hajimehoshi/ebiten/v2/internal/graphics"
+	"github.com/hajimehoshi/ebiten/v2/internal/graphicscommand"
+)
+
+type pixelsRecord struct {
+	rect image.Rectangle
+	pix  *graphics.ManagedBytes
+}
+
+func (p *pixelsRecord) readPixels(pixels []byte, region image.Rectangle, imageWidth, imageHeight int) {
+	r := p.rect.Intersect(region.Intersect(image.Rect(0, 0, imageWidth, imageHeight)))
+	if r.Empty() {
+		return
+	}
+
+	dstBaseX := r.Min.X - region.Min.X
+	dstBaseY := r.Min.Y - region.Min.Y
+	lineWidth := 4 * r.Dx()
+	if p.pix != nil {
+		srcBaseX := r.Min.X - p.rect.Min.X
+		srcBaseY := r.Min.Y - p.rect.Min.Y
+		for j := 0; j < r.Dy(); j++ {
+			dstX := 4 * ((dstBaseY+j)*region.Dx() + dstBaseX)
+			srcX := 4 * ((srcBaseY+j)*p.rect.Dx() + srcBaseX)
+			p.pix.Read(pixels[dstX:dstX+lineWidth], srcX, srcX+lineWidth)
+		}
+	} else {
+		for j := 0; j < r.Dy(); j++ {
+			dstX := 4 * ((dstBaseY+j)*region.Dx() + dstBaseX)
+			for i := 0; i < lineWidth; i++ {
+				pixels[i+dstX] = 0
+			}
+		}
+	}
+}
+
+type pixelsRecords struct {
+	records []*pixelsRecord
+}
+
+func (pr *pixelsRecords) addOrReplace(pixels *graphics.ManagedBytes, region image.Rectangle) {
+	if pixels.Len() != 4*region.Dx()*region.Dy() {
+		msg := fmt.Sprintf("restorable: len(pixels) must be 4*%d*%d = %d but %d", region.Dx(), region.Dy(), 4*region.Dx()*region.Dy(), pixels.Len())
+		if pixels == nil {
+			msg += " (nil)"
+		}
+		panic(msg)
+	}
+
+	// Remove or update the duplicated records first.
+	var n int
+	for _, r := range pr.records {
+		if r.rect.In(region) {
+			continue
+		}
+		pr.records[n] = r
+		n++
+	}
+	for i := n; i < len(pr.records); i++ {
+		pr.records[i] = nil
+	}
+	pr.records = pr.records[:n]
+
+	// Add the new record.
+	pr.records = append(pr.records, &pixelsRecord{
+		rect: region,
+		pix:  pixels,
+	})
+}
+
+func (pr *pixelsRecords) clear(region image.Rectangle) {
+	if region.Empty() {
+		return
+	}
+
+	var n int
+	var needsClear bool
+	for _, r := range pr.records {
+		if r.rect.In(region) {
+			continue
+		}
+		if r.rect.Overlaps(region) {
+			needsClear = true
+		}
+		pr.records[n] = r
+		n++
+	}
+	for i := n; i < len(pr.records); i++ {
+		pr.records[i] = nil
+	}
+	pr.records = pr.records[:n]
+	if needsClear {
+		pr.records = append(pr.records, &pixelsRecord{
+			rect: region,
+		})
+	}
+}
+
+func (pr *pixelsRecords) readPixels(pixels []byte, region image.Rectangle, imageWidth, imageHeight int) {
+	for i := range pixels {
+		pixels[i] = 0
+	}
+	for _, r := range pr.records {
+		r.readPixels(pixels, region, imageWidth, imageHeight)
+	}
+}
+
+func (pr *pixelsRecords) apply(img *graphicscommand.Image) {
+	// TODO: Isn't this too heavy? Can we merge the operations?
+	for _, r := range pr.records {
+		if r.pix != nil {
+			// Clone a ManagedBytes as the package graphicscommand has a different lifetime management.
+			img.WritePixels(r.pix.Clone(), r.rect)
+		} else {
+			clearImage(img, r.rect)
+		}
+	}
+}
+
+func (pr *pixelsRecords) appendRegions(regions []image.Rectangle) []image.Rectangle {
+	for _, r := range pr.records {
+		if r.rect.Empty() {
+			continue
+		}
+		regions = append(regions, r.rect)
+	}
+	return regions
+}
+
+func (pr *pixelsRecords) dispose() {
+	for _, r := range pr.records {
+		if r.pix == nil {
+			continue
+		}
+		// As the package graphicscommands already has cloned ManagedBytes objects, it is OK to release it.
+		_, f := r.pix.GetAndRelease()
+		f()
+	}
+}

internal/restorable/shader.go

@@ -0,0 +1,96 @@
+// Copyright 2020 The Ebiten Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package restorable
+
+import (
+	"fmt"
+
+	"golang.org/x/sync/errgroup"
+
+	"github.com/hajimehoshi/ebiten/v2/internal/builtinshader"
+	"github.com/hajimehoshi/ebiten/v2/internal/graphics"
+	"github.com/hajimehoshi/ebiten/v2/internal/graphicscommand"
+	"github.com/hajimehoshi/ebiten/v2/internal/shaderir"
+)
+
+type Shader struct {
+	shader *graphicscommand.Shader
+	ir     *shaderir.Program
+}
+
+func NewShader(ir *shaderir.Program) *Shader {
+	s := &Shader{
+		shader: graphicscommand.NewShader(ir),
+		ir:     ir,
+	}
+	theImages.addShader(s)
+	return s
+}
+
+func (s *Shader) Dispose() {
+	theImages.removeShader(s)
+	s.shader.Dispose()
+	s.shader = nil
+	s.ir = nil
+}
+
+func (s *Shader) restore() {
+	s.shader = graphicscommand.NewShader(s.ir)
+}
+
+func (s *Shader) Unit() shaderir.Unit {
+	return s.ir.Unit
+}
+
+var (
+	NearestFilterShader *Shader
+	LinearFilterShader  *Shader
+	clearShader         *Shader
+)
+
+func init() {
+	var wg errgroup.Group
+	var nearestIR, linearIR, clearIR *shaderir.Program
+	wg.Go(func() error {
+		ir, err := graphics.CompileShader([]byte(builtinshader.ShaderSource(builtinshader.FilterNearest, builtinshader.AddressUnsafe, false)))
+		if err != nil {
+			return fmt.Errorf("restorable: compiling the nearest shader failed: %w", err)
+		}
+		nearestIR = ir
+		return nil
+	})
+	wg.Go(func() error {
+		ir, err := graphics.CompileShader([]byte(builtinshader.ShaderSource(builtinshader.FilterLinear, builtinshader.AddressUnsafe, false)))
+		if err != nil {
+			return fmt.Errorf("restorable: compiling the linear shader failed: %w", err)
+		}
+		linearIR = ir
+		return nil
+	})
+	wg.Go(func() error {
+		ir, err := graphics.CompileShader([]byte(builtinshader.ClearShaderSource))
+		if err != nil {
+			return fmt.Errorf("restorable: compiling the clear shader failed: %w", err)
+		}
+		clearIR = ir
+		return nil
+	})
+	if err := wg.Wait(); err != nil {
+		panic(err)
+	}
+	NearestFilterShader = NewShader(nearestIR)
+	LinearFilterShader = NewShader(linearIR)
+	clearShader = NewShader(clearIR)
+}

internal/restorable/shader_test.go

@@ -0,0 +1,200 @@
+// Copyright 2018 The Ebiten Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package restorable_test
+
+import (
+	"image"
+	"image/color"
+	"testing"
+
+	"github.com/hajimehoshi/ebiten/v2/internal/graphics"
+	"github.com/hajimehoshi/ebiten/v2/internal/graphicsdriver"
+	"github.com/hajimehoshi/ebiten/v2/internal/restorable"
+	etesting "github.com/hajimehoshi/ebiten/v2/internal/testing"
+	"github.com/hajimehoshi/ebiten/v2/internal/ui"
+)
+
+func clearImage(img *restorable.Image, w, h int) {
+	emptyImage := restorable.NewImage(3, 3, restorable.ImageTypeRegular)
+	defer emptyImage.Dispose()
+
+	dx0 := float32(0)
+	dy0 := float32(0)
+	dx1 := float32(w)
+	dy1 := float32(h)
+	sx0 := float32(1)
+	sy0 := float32(1)
+	sx1 := float32(2)
+	sy1 := float32(2)
+	vs := []float32{
+		dx0, dy0, sx0, sy0, 0, 0, 0, 0,
+		dx1, dy0, sx1, sy0, 0, 0, 0, 0,
+		dx0, dy1, sx0, sy1, 0, 0, 0, 0,
+		dx1, dy1, sx1, sy1, 0, 0, 0, 0,
+	}
+	is := graphics.QuadIndices()
+	dr := image.Rect(0, 0, w, h)
+	img.DrawTriangles([graphics.ShaderSrcImageCount]*restorable.Image{emptyImage}, vs, is, graphicsdriver.BlendClear, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, restorable.NearestFilterShader, nil, graphicsdriver.FillRuleFillAll)
+}
+
+func TestShader(t *testing.T) {
+	img := restorable.NewImage(1, 1, restorable.ImageTypeRegular)
+	defer img.Dispose()
+
+	s := restorable.NewShader(etesting.ShaderProgramFill(0xff, 0, 0, 0xff))
+	dr := image.Rect(0, 0, 1, 1)
+	img.DrawTriangles([graphics.ShaderSrcImageCount]*restorable.Image{}, quadVertices(1, 1, 0, 0), graphics.QuadIndices(), graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, s, nil, graphicsdriver.FillRuleFillAll)
+
+	if err := restorable.ResolveStaleImages(ui.Get().GraphicsDriverForTesting()); err != nil {
+		t.Fatal(err)
+	}
+	if err := restorable.RestoreIfNeeded(ui.Get().GraphicsDriverForTesting()); err != nil {
+		t.Fatal(err)
+	}
+
+	want := color.RGBA{R: 0xff, A: 0xff}
+	got := pixelsToColor(img.BasePixelsForTesting(), 0, 0, 1, 1)
+	if !sameColors(got, want, 1) {
+		t.Errorf("got %v, want %v", got, want)
+	}
+}
+
+func TestShaderChain(t *testing.T) {
+	const num = 10
+	imgs := []*restorable.Image{}
+	for i := 0; i < num; i++ {
+		img := restorable.NewImage(1, 1, restorable.ImageTypeRegular)
+		defer img.Dispose()
+		imgs = append(imgs, img)
+	}
+
+	imgs[0].WritePixels(bytesToManagedBytes([]byte{0xff, 0, 0, 0xff}), image.Rect(0, 0, 1, 1))
+
+	s := restorable.NewShader(etesting.ShaderProgramImages(1))
+	for i := 0; i < num-1; i++ {
+		dr := image.Rect(0, 0, 1, 1)
+		imgs[i+1].DrawTriangles([graphics.ShaderSrcImageCount]*restorable.Image{imgs[i]}, quadVertices(1, 1, 0, 0), graphics.QuadIndices(), graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, s, nil, graphicsdriver.FillRuleFillAll)
+	}
+
+	if err := restorable.ResolveStaleImages(ui.Get().GraphicsDriverForTesting()); err != nil {
+		t.Fatal(err)
+	}
+	if err := restorable.RestoreIfNeeded(ui.Get().GraphicsDriverForTesting()); err != nil {
+		t.Fatal(err)
+	}
+
+	for i, img := range imgs {
+		want := color.RGBA{R: 0xff, A: 0xff}
+		got := pixelsToColor(img.BasePixelsForTesting(), 0, 0, 1, 1)
+		if !sameColors(got, want, 1) {
+			t.Errorf("%d: got %v, want %v", i, got, want)
+		}
+	}
+}
+
+func TestShaderMultipleSources(t *testing.T) {
+	var srcs [graphics.ShaderSrcImageCount]*restorable.Image
+	for i := range srcs {
+		srcs[i] = restorable.NewImage(1, 1, restorable.ImageTypeRegular)
+	}
+	srcs[0].WritePixels(bytesToManagedBytes([]byte{0x40, 0, 0, 0xff}), image.Rect(0, 0, 1, 1))
+	srcs[1].WritePixels(bytesToManagedBytes([]byte{0, 0x80, 0, 0xff}), image.Rect(0, 0, 1, 1))
+	srcs[2].WritePixels(bytesToManagedBytes([]byte{0, 0, 0xc0, 0xff}), image.Rect(0, 0, 1, 1))
+
+	dst := restorable.NewImage(1, 1, restorable.ImageTypeRegular)
+
+	s := restorable.NewShader(etesting.ShaderProgramImages(3))
+	dr := image.Rect(0, 0, 1, 1)
+	dst.DrawTriangles(srcs, quadVertices(1, 1, 0, 0), graphics.QuadIndices(), graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, s, nil, graphicsdriver.FillRuleFillAll)
+
+	// Clear one of the sources after DrawTriangles. dst should not be affected.
+	clearImage(srcs[0], 1, 1)
+
+	if err := restorable.ResolveStaleImages(ui.Get().GraphicsDriverForTesting()); err != nil {
+		t.Fatal(err)
+	}
+	if err := restorable.RestoreIfNeeded(ui.Get().GraphicsDriverForTesting()); err != nil {
+		t.Fatal(err)
+	}
+
+	want := color.RGBA{R: 0x40, G: 0x80, B: 0xc0, A: 0xff}
+	got := pixelsToColor(dst.BasePixelsForTesting(), 0, 0, 1, 1)
+	if !sameColors(got, want, 1) {
+		t.Errorf("got %v, want %v", got, want)
+	}
+}
+
+func TestShaderMultipleSourcesOnOneTexture(t *testing.T) {
+	src := restorable.NewImage(3, 1, restorable.ImageTypeRegular)
+	src.WritePixels(bytesToManagedBytes([]byte{
+		0x40, 0, 0, 0xff,
+		0, 0x80, 0, 0xff,
+		0, 0, 0xc0, 0xff,
+	}), image.Rect(0, 0, 3, 1))
+	srcs := [graphics.ShaderSrcImageCount]*restorable.Image{src, src, src}
+
+	dst := restorable.NewImage(1, 1, restorable.ImageTypeRegular)
+
+	s := restorable.NewShader(etesting.ShaderProgramImages(3))
+	dr := image.Rect(0, 0, 1, 1)
+	srcRegions := [graphics.ShaderSrcImageCount]image.Rectangle{
+		image.Rect(0, 0, 1, 1),
+		image.Rect(1, 0, 2, 1),
+		image.Rect(2, 0, 3, 1),
+	}
+	dst.DrawTriangles(srcs, quadVertices(1, 1, 0, 0), graphics.QuadIndices(), graphicsdriver.BlendCopy, dr, srcRegions, s, nil, graphicsdriver.FillRuleFillAll)
+
+	// Clear one of the sources after DrawTriangles. dst should not be affected.
+	clearImage(srcs[0], 3, 1)
+
+	if err := restorable.ResolveStaleImages(ui.Get().GraphicsDriverForTesting()); err != nil {
+		t.Fatal(err)
+	}
+	if err := restorable.RestoreIfNeeded(ui.Get().GraphicsDriverForTesting()); err != nil {
+		t.Fatal(err)
+	}
+
+	want := color.RGBA{R: 0x40, G: 0x80, B: 0xc0, A: 0xff}
+	got := pixelsToColor(dst.BasePixelsForTesting(), 0, 0, 1, 1)
+	if !sameColors(got, want, 1) {
+		t.Errorf("got %v, want %v", got, want)
+	}
+}
+
+func TestShaderDispose(t *testing.T) {
+	img := restorable.NewImage(1, 1, restorable.ImageTypeRegular)
+	defer img.Dispose()
+
+	s := restorable.NewShader(etesting.ShaderProgramFill(0xff, 0, 0, 0xff))
+	dr := image.Rect(0, 0, 1, 1)
+	img.DrawTriangles([graphics.ShaderSrcImageCount]*restorable.Image{}, quadVertices(1, 1, 0, 0), graphics.QuadIndices(), graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, s, nil, graphicsdriver.FillRuleFillAll)
+
+	// Dispose the shader. This should invalidate all the images using this shader i.e., all the images become
+	// stale.
+	s.Dispose()
+
+	if err := restorable.ResolveStaleImages(ui.Get().GraphicsDriverForTesting()); err != nil {
+		t.Fatal(err)
+	}
+	if err := restorable.RestoreIfNeeded(ui.Get().GraphicsDriverForTesting()); err != nil {
+		t.Fatal(err)
+	}
+
+	want := color.RGBA{R: 0xff, A: 0xff}
+	got := pixelsToColor(img.BasePixelsForTesting(), 0, 0, 1, 1)
+	if !sameColors(got, want, 1) {
+		t.Errorf("got %v, want %v", got, want)
+	}
+}

internal/ui/context.go

@@ -180,6 +180,12 @@ func (c *context) newOffscreenImage(w, h int) *Image {
 }
 
 func (c *context) drawGame(graphicsDriver graphicsdriver.Graphics, ui *UserInterface, forceDraw bool) error {
+	if (c.offscreen.imageType == atlas.ImageTypeVolatile) != ui.IsScreenClearedEveryFrame() {
+		w, h := c.offscreen.width, c.offscreen.height
+		c.offscreen.Deallocate()
+		c.offscreen = c.newOffscreenImage(w, h)
+	}
+
 	// isOffscreenModified is updated when an offscreen's modifyCallback.
 	c.isOffscreenModified = false
 

internal/ui/image.go

@@ -15,6 +15,7 @@
 package ui
 
 import (
+	"fmt"
 	"image"
 	"math"
 
@@ -85,7 +86,16 @@ func (i *Image) DrawTriangles(srcs [graphics.ShaderSrcImageCount]*Image, vertice
 
 	if antialias {
 		if i.bigOffscreenBuffer == nil {
-			i.bigOffscreenBuffer = i.ui.newBigOffscreenImage(i, atlas.ImageTypeUnmanaged)
+			var imageType atlas.ImageType
+			switch i.imageType {
+			case atlas.ImageTypeRegular, atlas.ImageTypeUnmanaged:
+				imageType = atlas.ImageTypeUnmanaged
+			case atlas.ImageTypeScreen, atlas.ImageTypeVolatile:
+				imageType = atlas.ImageTypeVolatile
+			default:
+				panic(fmt.Sprintf("ui: unexpected image type: %d", imageType))
+			}
+			i.bigOffscreenBuffer = i.ui.newBigOffscreenImage(i, imageType)
 		}
 
 		i.bigOffscreenBuffer.drawTriangles(srcs, vertices, indices, blend, dstRegion, srcRegions, shader, uniforms, fillRule, canSkipMipmap)

internal/ui/ui.go

@@ -181,6 +181,7 @@ type RunOptions struct {
 	ColorSpace               graphicsdriver.ColorSpace
 	X11ClassName             string
 	X11InstanceName          string
+	StrictContextRestoration bool
 }
 
 // InitialWindowPosition returns the position for centering the given second width/height pair within the first width/height pair.

internal/ui/ui_mobile.go

@@ -28,6 +28,7 @@ import (
 	"github.com/hajimehoshi/ebiten/v2/internal/graphicscommand"
 	"github.com/hajimehoshi/ebiten/v2/internal/graphicsdriver"
 	"github.com/hajimehoshi/ebiten/v2/internal/hook"
+	"github.com/hajimehoshi/ebiten/v2/internal/restorable"
 )
 
 var (
@@ -98,7 +99,10 @@ type userInterfaceImpl struct {
 	touches    []TouchForInput
 
 	fpsMode  atomic.Int32
-	renderRequester RenderRequester
+	renderer Renderer
+
+	strictContextRestoration     bool
+	strictContextRestorationOnce sync.Once
 
 	m sync.RWMutex
 }
@@ -152,6 +156,11 @@ func (u *UserInterface) runMobile(game Game, options *RunOptions) (err error) {
 	u.graphicsDriver = g
 	u.setGraphicsLibrary(lib)
 	close(u.graphicsLibraryInitCh)
+	u.strictContextRestoration = options.StrictContextRestoration
+	if !u.strictContextRestoration {
+		restorable.Disable()
+	}
+	u.renderer.SetStrictContextRestoration(u.strictContextRestoration)
 
 	for {
 		if err := u.update(); err != nil {
@@ -239,10 +248,10 @@ func (u *UserInterface) SetFPSMode(mode FPSModeType) {
 }
 
 func (u *UserInterface) updateExplicitRenderingModeIfNeeded(fpsMode FPSModeType) {
-	if u.renderRequester == nil {
+	if u.renderer == nil {
 		return
 	}
-	u.renderRequester.SetExplicitRenderingMode(fpsMode == FPSModeVsyncOffMinimum)
+	u.renderer.SetExplicitRenderingMode(fpsMode == FPSModeVsyncOffMinimum)
 }
 
 func (u *UserInterface) readInputState(inputState *InputState) {
@@ -297,23 +306,24 @@ func (u *UserInterface) Monitor() *Monitor {
 func (u *UserInterface) UpdateInput(keys map[Key]struct{}, runes []rune, touches []TouchForInput) {
 	u.updateInputStateFromOutside(keys, runes, touches)
 	if FPSModeType(u.fpsMode.Load()) == FPSModeVsyncOffMinimum {
-		u.renderRequester.RequestRenderIfNeeded()
+		u.renderer.RequestRenderIfNeeded()
 	}
 }
 
-type RenderRequester interface {
+type Renderer interface {
 	SetExplicitRenderingMode(explicitRendering bool)
+	SetStrictContextRestoration(strictContextRestoration bool)
 	RequestRenderIfNeeded()
 }
 
-func (u *UserInterface) SetRenderRequester(renderRequester RenderRequester) {
+func (u *UserInterface) SetRenderer(renderer Renderer) {
-	u.renderRequester = renderRequester
+	u.renderer = renderer
 	u.updateExplicitRenderingModeIfNeeded(FPSModeType(u.fpsMode.Load()))
 }
 
 func (u *UserInterface) ScheduleFrame() {
-	if u.renderRequester != nil && FPSModeType(u.fpsMode.Load()) == FPSModeVsyncOffMinimum {
+	if u.renderer != nil && FPSModeType(u.fpsMode.Load()) == FPSModeVsyncOffMinimum {
-		u.renderRequester.RequestRenderIfNeeded()
+		u.renderer.RequestRenderIfNeeded()
 	}
 }
 

mobile/ebitenmobileview/mobile.go

@@ -30,6 +30,7 @@ import (
 	"sync"
 
 	"github.com/hajimehoshi/ebiten/v2"
+	"github.com/hajimehoshi/ebiten/v2/internal/restorable"
 	"github.com/hajimehoshi/ebiten/v2/internal/ui"
 )
 
@@ -112,17 +113,20 @@ func Resume() error {
 	return ui.Get().SetForeground(true)
 }
 
+func OnContextLost() {
+	restorable.OnContextLost()
+}
+
 func DeviceScale() float64 {
 	return ui.Get().Monitor().DeviceScaleFactor()
 }
 
-type RenderRequester interface {
+type Renderer interface {
-	SetExplicitRenderingMode(explicitRendering bool)
+	ui.Renderer
-	RequestRenderIfNeeded()
 }
 
-func SetRenderRequester(renderRequester RenderRequester) {
+func SetRenderer(renderer Renderer) {
-	ui.Get().SetRenderRequester(renderRequester)
+	ui.Get().SetRenderer(renderer)
 }
 
 func SetSetGameNotifier(setGameNotifier SetGameNotifier) {

run.go

@@ -297,6 +297,24 @@ type RunGameOptions struct {
 
 	// X11InstanceName is an instance name in the ICCCM WM_CLASS window property.
 	X11InstanceName string
+
+	// StrictContextRestration indicates whether the context lost should be restored strictly by Ebitengine or not.
+	//
+	// StrictContextRestration is available only on Android. Otherwise, StrictContextRestration is ignored.
+	//
+	// When StrictContextRestration is false, Ebitengine tries to rely on the OS to restore the context.
+	// In Android, Ebitengien uses `GLSurfaceView`'s `setPreserveEGLContextOnPause(true)`.
+	// This works in most cases, but it is still possible that the context is lost in some minor cases.
+	// With StrictContextRestration false, the activity's launch mode should be singleInstance,
+	// or the activity no longer works correctly after the context is lost.
+	//
+	// When StrictContextRestration is true, Ebitengine tries to restore the context more strictly.
+	// This is useful when you want to restore the context in any case.
+	// However, this might cause a performance issue since Ebitengine tries to keep all the information
+	// to restore the context.
+	//
+	// The default (zero) value is false.
+	StrictContextRestration bool
 }
 
 // RunGameWithOptions starts the main loop and runs the game with the specified options.
@@ -725,6 +743,7 @@ func toUIRunOptions(options *RunGameOptions) *ui.RunOptions {
 		ColorSpace:               graphicsdriver.ColorSpace(options.ColorSpace),
 		X11ClassName:             options.X11ClassName,
 		X11InstanceName:          options.X11InstanceName,
+		StrictContextRestoration: options.StrictContextRestration,
 	}
 }